1. Field of the Invention
The present invention relates to multilayer substrates, and more particularly to multilayer substrates having improved electrical breakdown resistance.
2. Description of the Related Art
In general, a multilayer substrate used in electronic equipment is provided with several patterns for control circuits or electric power circuits, and in some cases, a large potential difference may be generated between such circuit patterns.
Conventional multilayer substrates have problems related to their electrical insulation breakdown voltage.
A large potential difference is generated, for example, in a multilayer substrate used for a switching power source. In the following, a multilayer substrate used for a switching power source will be illustrated.
In a switching power source, a switching transformer is used. For a high switching frequency of around several hundred kilohertz, a thin and small transformer comprising a lamination of coil patterns is used as a switching transformer. In some cases of an even higher switching frequency, the transformer is formed within a multilayer substrate since the inductance of the transformer may be small.
Typically, such a transformer comprises a primary coil and a secondary coil, and the primary coil is connected to an input circuit pattern of the switching power source while the secondary coil is connected to an output circuit pattern. In general, a voltage 2 to 3 times the power source voltage is applied to a transformer of a switching power source, and therefore, the potential difference between the coil patterns of the primary coil and the secondary coil in the transformer is extremely large.
As shown in FIGS. 5 and 6, a multilayer substrate used in such a switching power source comprises, for example, substrates 1, 2, 3 and 4, circuit patterns 8 to 15, and preimpregs 5, 6 and 7 which fill the spaces between the substrates and comprise glass fabric bases 5a, 6a and 7a and epoxy resin portions 16, 17 and 18, respectively.
On the front or rear surfaces of the substrates 1, 2, 3 and 4, circuit patterns 8 to 15 including coil patterns and wiring patterns are formed. Further, the substrates 1, 2, 3 and 4 are laminated with the intervention of the insulating preimpreg layers 5, 6, and 7, which respectively comprise glass fabric bases 5a, 6a and 7a and epoxy resin portions 16, 17 and 18 which are cured while combined with the glass fabric bases 5a, 6a and 7a.
For each substrate 1, 2, 3 or 4, a glass-epoxy laminate plate or the like is used which may have a thickness of, for example, 100 .mu.m. Further, each of the circuit patterns 8 to 15 is formed, for example, by etching copper foil which is deposited on the front or rear surface of the substrate 1, 2, 3, or 4 and has a thickness of around 35 to 70 .mu.m. As occasion demands, the circuit patterns formed on the front or rear surfaces of the substrates 1, 2, 3 and 4 are connected by providing through-holes. The coil patterns are connected in series to the coil patterns in other laminated layers through through-holes in order to increase the turn number of the resulting coil. The turn number of the coil is determined depending on the intended properties of the transformer.
With reference to FIG. 5, a method for producing a multilayer substrate will be schematically. illustrated below.
Substrates 1, 2, 3 and 4 having circuit patterns 8 to 15 formed on their front or rear surfaces and preimpregs 5, 6 and 7 are alternately laminated, respectively. The preimpregs 5, 6 and 7 are thin sheets respectively comprising glass fabric bases 5a, 6a and 7a in which an epoxy resin is incorporated and half cured.
The thus laminated substrates 1, 2, 3 and 4 and preimpregs 5, 6 and 7 are pressed in the direction of lamination, and the whole laminate is heated. As a result, the epoxy resin is melted and then cured, so that the substrates 1, 2, 3 and 4 adhere one upon another, and thus a multilayer substrate is formed, as shown in FIG. 6. Hereupon, since the epoxy resin wraps the circuit pattern 9 to 14 inside the laminate and fills the spaces between the substrates 1, 2, 3 and 4, electrical insulation therebetween is secured.
The above-mentioned problems with electrical insulation breakdown voltage are believed to arise because of the apparatus and methods used for manufacturing the conventional multilayer substrate.
In general, in order to achieve such a multilayer structure, a so-called vacuum lamination-molding molding press machine is used. In this apparatus, the pressing part can be entirely sealed for ambient pressure reduction, whereby pressing and heating can be carried out under a reduced pressure.
More specifically, substrates and preimpregs to be laminated are fixed in such an apparatus by a jig, the entire pressing part of the apparatus is then sealed and depressurized, and the substrates and preimpregs are heated to approximately 170 to 180.degree. C. and pressed in the direction of lamination under a pressure of 30 to 40 kg/cm.sup.2. The degree of vacuum in the depressurized pressing part is set at 13,332 Pa or less, and heating is performed for 70 to 90 min. while pressing is performed for 15 to 20 min.
The epoxy resin included in the preimpregs exhibits a minimum melt-viscosity at around 130.degree. C. in the heating step, and starts to harden above such a temperature. Due to these characteristics, the epoxy resin having a viscosity lowered by heating frequently leaks out from the spaces between the substrates during lamination of the substrates. As a result, voids are generated between the substrates, and the insulation breakdown voltage between the layers is thereby lowered.
In particular, as shown in FIG. 7, when such a void 19 is generated in an insulating layer 22 between coil patterns of a primary coil 20 and a secondary coil 21 in a transformer which requires a high electrical insulation breakdown voltage, the electrical insulation breakdown voltage between the coil patterns may be unsatisfactory.
A similar reduction in electrical insulation breakdown voltage may also occur between a pattern of a power source circuit at a high potential and a pattern of a control circuit at a low potential which are connected to the coil patterns.
Various attempts have been made to solve the above-described problems. For example, for manufacturing the pair of substrates 23 and 24 shown in FIG. 7, a rectangular-frame-shaped pattern (not shown) is formed on the rear surface of the substrate 23 and the front surface of the substrate 24 along the peripheries of the substrates 23 and 24 so as to surround all the circuit patterns formed thereon, for preventing leakage of the melted epoxy resin from between the substrates 23 and 24. The known leakage preventing pattern is not part of the circuit patterns in the multilayer substrate as it is removed after the substrates are laminate.
The known leakage preventing pattern can prevent void generation in the case of a small-size multilayer substrate. However, when a multilayer substrate is produced having a large size or including circuit patterns having complicated shapes, heat and pressure applied for production of the multilayer substrate are unevenly transferred into the component substrates. Due to this, the melted epoxy resin is rarely evenly distributed in the spaces between the component substrates, and local voids are readily generated. In particular, when such a void is generated between coil patterns in a transformer exhibiting an extremely large potential difference, or between a high-potential circuit pattern and a low-potential circuit pattern, the electrical insulation breakdown voltage of the multilayer substrate is unsatisfactory.